Re: [PATCH] KVM: arm64: Always check the state from hyp_ack_unshare()

[Quentin Perret <qperret@google.com>]

On Thursday 28 Nov 2024 at 15:44:06 (+0000), Quentin Perret wrote:
> There are multiple pKVM memory transitions where the state of a page is
> not cross-checked from the completer's PoV for performance reasons.
> For example, if a page is PKVM_PAGE_OWNED from the initiator's PoV,
> we should be guaranteed by construction that it is PKVM_NOPAGE for
> everybody else, hence allowing us to save a page-table lookup.
> 
> When it was introduced, hyp_ack_unshare() followed that logic and bailed
> out without checking the PKVM_PAGE_SHARED_BORROWED state in the
> hypervisor's stage-1. This was correct as we could safely assume that
> all host-initiated shares were directed at the hypervisor at the time.
> But with the introduction of other types of shares (e.g. for FF-A or
> non-protected guests), it is now very much required to cross check this
> state to prevent the host from running __pkvm_host_unshare_hyp() on a
> page shared with TZ or a non-protected guest.
> 
> Thankfully, if an attacker were to try this, the hyp_unmap() call from
> hyp_complete_unshare() would fail, hence causing to WARN() from
> __do_unshare() with the host lock held, which is fatal. But this is
> fragile at best, and can hardly be considered a security measure.
> 
> Let's just do the right thing and always check the state from
> hyp_ack_unshare().
> 
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index caba3e4bd09e..e75374d682f4 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -783,9 +783,6 @@ static int hyp_ack_unshare(u64 addr, const struct pkvm_mem_transition *tx)
>  	if (tx->initiator.id == PKVM_ID_HOST && hyp_page_count((void *)addr))
>  		return -EBUSY;
>  
> -	if (__hyp_ack_skip_pgtable_check(tx))
> -		return 0;
> -
>  	return __hyp_check_page_state_range(addr, size,
>  					    PKVM_PAGE_SHARED_BORROWED);
>  }
> -- 
> 2.47.0.338.g60cca15819-goog

Shameless inbox bump for this one :-)

It should hopefully be a fairly straightforward fix.

Thanks,
Quentin